The present invention relates generally to exposure apparatuses, and more particularly to an exposure apparatus that is used to expose an object, such as a single crystal plate for a semiconductor wafer, a glass plate for a liquid crystal display (“LCD”), and the like, an optical stop unit used for such an exposure apparatus, a device fabrication method using the exposed object, and a device fabricated from the exposed object. The present invention is suitable, for example, for an exposure apparatus that exposes a single crystal plate for a semiconductor wafer in a step-and-scan, scan, or step-and-repeat projection manner in a photolithography process.
The “step-and-scan” manner, as used herein, is one mode of exposure method which exposes a pattern on a mask onto a wafer by continuously scanning the wafer relative to the mask or reticle (these terms are used interchangeably in this application) and moving, after a shot of exposure, the wafer stepwise to the next exposure area to be shot. The “scan” manner is another mode of exposure method which uses a projection optical system to project part of a mask pattern onto a wafer, and exposes the entire mask pattern to the wafer by relatively and synchronously scanning the mask and the object relative to the projection optical system. The “step-and-repeat” manner is still another mode of exposure method which moves a wafer stepwise to an exposure area for the next shot every shot of cell projection onto the wafer.
Along with recent demands on smaller and lower profile electronic devices, fine semiconductor devices to be mounted onto these electronic devices have been increasingly demanded. For example, a design rule for a pattern on a mask attempts a line and space (L & S) of 130 nm on a mass production line, which will be predictably increasingly smaller in the future. L & S denotes an image projected to a wafer in exposure with equal line and space widths, and serves as an index of exposure resolution. The exposure has three important factors including resolution, overlay accuracy, and throughput. The resolution is the minimum size for a precise pattern transfer. The overlay accuracy is a precision with which to overlay multiple patterns over an object to be exposed. The throughput is the number of sheets exposed per unit of time.
There are two basic exposure methods including a full-size transfer method and a projection method. The full-size transfer includes a contact method that brings a mask into close contact with an object to be exposed, and a proximity method that slightly spaces them from each other. Although the contact method may provide higher resolution, dusts and silicon fragments enter under a mask in a compressed state, and damage the mask, causing the exposed object to be flawed and defective. The proximity method ameliorates such problems, but still possibly damages the mask if a distance between the mask and the object to be exposed becomes shorter than the maximum size of a dust particle.
A projection method has been suggested accordingly which farther spaces the mask from the object to be exposed. Among projection modes, a scan projection exposure apparatus has been the recent trend for the improved resolution and expanded exposure area, which exposes the entire mask pattern onto the wafer by exposing part of the mask, synchronizing the mask and wafer, and scanning the wafer continuously or intermittently.
A projection exposure apparatus generally includes an illumination optical system that uses light emitted from a light source to illuminate a mask, and a projection optical system located between the mask and the object to be exposed. For a uniform illumination area, the illumination optical system introduces the light from a light source to a light integrator, such as a fly-eye lens, which includes a plurality of rod lenses, and uses a condenser lens to Kohler-illuminate the mask surface with an exit plane of the light integrator as a secondary light source surface.
The following equation provides the resolution R of the projection exposure apparatus where λ is a light-source wavelength and NA is a numerical aperture of the projection optical system:
                    R        =                              k            1                    ·                      λ                          N              ⁢                                                          ⁢              A                                                          (        1        )            
The shorter the wavelength becomes and the higher the NA increases, the better the resolution thus becomes. For improved resolution, an exposure apparatus has recently been put to practical use that has a projection optical system with a higher NA, e.g., NA=0.70 or higher.
A focus range that maintains desired imaging performance is called a depth of focus (“DOF”), which is given by the following equation:
                    DOF        =                              k            2                    ·                      λ                          N              ⁢                                                          ⁢                              A                2                                                                        (        2        )            
The shorter the wavelength becomes and the higher the NA increases, the smaller the DOF thus becomes. A small DOF would make difficult focusing as well as requiring a flatter plate and a more accurate focusing. Therefore, a large DOF is basically desirable.
An actual process calculates a condition of an optimal NA to improve the resolution while securing the DOF. A projection optical system thus usually includes an optical stop unit for continuously varying NA, selecting and using the NA. For example, as disclosed in Japanese Laid-Open Patent Application 7-301845, a conventional NA-adjusting optical stop unit uses an iris stop unit that includes overlaying plural light blocking plates, and varies a diameter of an aperture by sliding and turning each light blocking plate.
For an enlarged chip size of a semiconductor device, a step-and-repeat exposure apparatus (stepper) is shifted to a step-and-scan exposure apparatus (scanner). The step-and-scan manner uses a slit-shaped exposure field to maximize the performance of a certain boundary in the projection optical system and expand an exposure area.
A large exposure area and an increased NA would enlarge an optical element (such as a lens's effective diameter of about Φ 200˜300 mm), and an aperture diameter of an optical stop unit included in an optical system. A large aperture of a stop diameter would enlarge a light blocking plate that makes up an iris stop unit and increase the number of light blocking plates. Since the light blocking plate is adapted to be as thin as possible so that it can shield light having various angles, a larger light blocking plate easily deteriorates its flatness. The deteriorated flatness of the light blocking plate would increase the load to a cam that drives the light blocking plate, causing the cam idle or dead, and uncontrollability of driving. In particular, an optical stop unit is usually housed in a lens barrel in a projection optical system inaccessibly, and any failure and malfunction in the optical stop unit would result in an extensive repair in the exposure apparatus, consequently lowering the economical efficiency of the exposure apparatus, and the throughput of the exposure due to the repair time.